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nite States
atent
ECC
3,032,447
Patented May I, 1962
2
1
3,032,447
?lm or the like. In accordance with this invention it has
been found that the undesirable metallic cations can be
continuously removed from an aqueous hexavalent
Werner Rausch, Frankfurt am Main, Germany, assignor
chromium-containing solution, on a continuous basis, by
the step of preliminarily conditioning the strongly acidic
to Parker Rust Proof Company, Detroit, Mich., a cor
cation exchange resin so that the effluent from the resin
PROCESS FOR CONTINUOUSLY OPERATING
CHROMATING SOLUTIONS
poration of Michigan
No Drawing. Filed Nov. 26, 1958, Ser. No. 776,443
Claims priority, application Germanyv Nov. 28, 1957
4 Claims. (Cl. 148-62)
bed has substantially the same pH as that possessed by
the solution fed into the ion exchange resin bed. It
has been further found that an aqueous hexavalent
10 chromium-containing solution is not rendered less ef?cient
in its metal coating-forming ability by the presence there
This invention relates to improvements in the continu
in of the alkali metal ions, including the ammonium ion.
ous operation of chromating processes and more par
The preliminary conditioning of the strongly acidic
ticularly relates to improvements in the continuous opera
cation exchange resin bed of this invention is most easily
tion of chromating solutions to form protective coatings
on metals such as zinc, aluminum, magnesium, cadmium, 15 accomplished by preliminarily passing a portion of the
operating chromating solution, or a separate chromating
copper, iron, brass, bronze and the like.
solution of substantially equivalent acidity to that of the
Chromating solutions, suitable for forming protective
operating solution, through the resin bed and continuing
coatings on metallic surfaces, contain as the principal
the passage of such solution through the bed until the
ingredient hexavalent chromium and have been modi?ed
to include a number of other ions to modify the rate 20 measured pH of the ef?uent is substantially the same as
the pH of the feed solution. The resin bed is then in
condition for treating portions of the operating chromat
of coating formation or the properties in the resulting
coating, such for example as ?uoride ion, complex fluo
ride ions, chloride ions, sulfate ions, nitrate ions, formate
ions, ferricyanide ions, etc. During the operation of
ing solution and the return of the ef?uent from such a
bed does not appreciably change the acidity of the operat
ing chromating solution and that solution maintains its
such an aqueous hexavalent chromium solution the hex
effectiveness as a metal coating solution.
avalent chromium is reduced to trivalent chromium and
the trivalent chromium tends to build up in concentra
tion in the bath. Additionally the metallic ions from the
surface being coated increase in concentration in the bath
and as the result of these increases the bath becomes
less acid, or the pH value of the bath increases with con
tinued use. Decreases in acidity, or increases in pH, re
duce the rate of metal coating formation and are unde
sirable, and for continuous operation it has been con
the resin bed itself. When the ion exchange resin bed
nevertheless gradully diminishes.
acidic regenerating solution will enable the regeneration
of the spent resin bed without requiring the repetition
The ratio of hydrogen ions to the other cations in the
e?iuent can be obtained by analysis of the effluent for
such ions, and this ratio is a measure of the same ions in
becomes depleted the bed is regenerated by modifying the
normally employed mineral acid regeneration treatment
in the following manner. A proportion of an alkali metal
ion is incorporated in the regenerating acid and this modi
ventional to maintain the desired pH condition by the 35 ?ed regenerating acidic solution is used to regenerate
the bed. After regeneration the relative proportion of
addition of acid. For continuous operation such solu
hydrogen and alkali metal ions in the regenerated resin
tions have been replenished by the addition of hexavalent
bed is checked to insure that it corresponds to substan
chromium compounds, desired modifying or activating
tially the proportions of those ions which exist in the
ions and the pH has been maintained at the desired value
by periodic additions of acid. With this type of operation 40 operating chromating solution. Based upon this analysis
and any necessary alteration of the proportion of alkali
the concentration of trivalent chromium and other unde
metal ions in the regenerating acid solution to make the
sirable metallic ions in the solution continue to build up
resulting ion exchange resin contain the desired propor
so that even though the pH is maintained, the hexavalent
tion of hydrogen and alkali metal ions, a regenerating
ion concentration and the activator ion concentrations are
preserved, the coating-forming ability of the solution 45 acidic solution is established. The use of this particular
In an attempt to overcome these disadvantages it has
of the treatment of that bed with a quantity of that oper
been proposed to remove the undesirable cations by the
ating solution to place it in the desired hydrogen ion
of the type which are capable of removing the undesir 50 alkali metal ion balanced condition. It will thus be ap
parent that an operating chromating solution can be main
able cations, namely the strongly acidic cation exchange
tained free of the undesired cations using such a modi?ed
resins. Such resins remove all metallic cations from the
use of ion exchange resins. The ion exchange resins are
ion exchange resin bed without substantially modifying
the acidity conditions in the operating chromating solu
chromating solution including trivalent chromium, the
metallic ion dissolved from the surface being coated such
as zinc, aluminum, ion, magnesium, copper, etc. and alkali
metal ions and replaces these ions with the hydrogen
ion from the ion exchange resin. The e?luent which is
returned from the ion exchange resin bed to the operat
ing chromating solution is therefore more acid than the
operating chromating solution.
tion. Thus in order to keep a particular chromating
solution in effective metal coating condition, it is neces
sary to determine only the ratio of hydrogen ions to alkali
metal ions in the operating solution and the appropriate
ratio of these same ions in the regenerating solution for
60 the resin bed and thereafter the resin bed can be alter
In. accordance with this invention it has been found
that continuous operation of chromating solutions em
ploying strongly acidic cation exchange resin beds for
nately employed to treat a portion of the operating chro
mating solution, on a continuous batch basis, and regener
ated with the modi?ed acidic regenerating solution.
For any particular chromating solution operation, the
the removal of the undesirable metallic cation introduces
an undesirable quantity of the hydrogen ion into the 65 most effective resin bed for use therewith can be initially
established in accordance with the above speci?ed pro
operating solution upon continued use. The proportion
cedure of determining the ratio of hydrogen ions to other
of hydrogen ion thus introduced into the operating
innocuous metallic cations, and additional resin beds can
chromating solution ultimately raises the acidity of the
operating solution to a point such that the resulting '
be formulated by admixing appropriate quantities of the
chromate coatings exhibit decreased e?ectiveness in pro 70 hydrogen form of strongly acidic cation exchange resins
and the sodium forms of such resins, if desired. As above
tecting the metal surface against corrosion, or in serving
indicated the chromating solutions of this invention may
as a base for paint or other overlayer such as a plastic
3,032,447
3
4
vary widely in their constituent materials and may con
tain any of the modifying or accelerating ions which are
known to enhance the coating-forming ability of aqueous
hexavalent chromium solutions. The‘ hexavalent chro
mium ion may be introduced into the solution as chromic
in the case of an aluminum chromating solution is greater
than can be tolerated in the case of a zinc chromating
solution before adverse effects are encountered in the
corrosion resistance of the resulting coating. In each case
the preferred condition is to avoid any change in the
acidity of the operating chromating solution as the result
acid or as a dichromate. It is preferred to employ the
chromic acid source since any metallic ion which is pres
ent in the chromate salt will be removed in the cation
of the return of the effluent to’ that solution, but as a
general guide it is satisfactory to control the pH of the
exchange resin treatment and this removal unnecessarily
eflluent, relative to the pH of the operating chromating
loads the ion exchange resin. It is therefore preferred 10 solution fed to the ion exchange resin bed within a small
to avoid the use of dichromate salts and particularly
range of pH, for. example, about plus or minus 0.5 pH.
heavy metal dichromate salts although such salts can be
used except for this particular disadvantage. For treat
ing aluminum the presence of the ?uoride ion is desirable
In every case the limiting condition is the control of the
pH of the e?luent ‘from the ion exchange resin bed to a
degree of acidity so close to that of the operating chro
and in such cases the ?uoride ion may be introduced as 15 mating solution that the coating-producing ability of the
hydro?uoric acid, ?uoboric acid, ?uosilicic acid, or the
solution and the corrosion protective value of the result
alkali metal salts thereof. The ferricyanide ion may be
ing coating is not detrimentally affected.
introduced in the form of the acid or as the alkali metal
In the operation of zinc chromating solutions it has
salt and for aluminum coating solutions a suitable propor
been found to be desirable to continuously control the
tion is between about 0.02% and about 0.2% by weight. 20 concentration of the trivalent chromium ions and the zinc
The cation exchange resins which have been found
ions in ‘the solution. It was found that the gradual in
suitable for the purposes of this invention are generally
crease in the concentration of the zinc ions in a zinc
designated the strongly acidic cation exchange resins such
chromating solution was harmful and the zinc ions could
as styrene-divinyl benzene resins which have been sulfo
not be rendered innocuous by the addition of acid an
mated with sulfuric acid. Such resins are commercially 25 nions, such as nitrate, even at concentrations as high as
available and one such resin which has been found to be
10-20 grams/liter of nitrate because such concentrations
satisfactory for the purposes of this invention is Dowex
make the coating powdery and easily wiped off. For con
50, which is available from the Dow Chemical Company.
tinuous operation of a zinc chromating solution the sum
Other similar resins are available under the designations
of the zinc ions and the trivalent chromium ions should
lR-l20 and Nalcite HGR. Such suitable resins are sul
be maintained below about 5 grams/liter and preferably
fonated styrene-divinyl benzene resins containing 1%
below 2 grams/liter, for example in the range of .2 to
16% divinyl benzene, preferably 4%-8% divinyl benzene
and having a mesh size between 20 and 100.
2 grams/liter.
The adverse effect of the increase in zinc ion concen
While it has been generally indicated above that the
alkali metal ions, including ammonium ions, represent
the type of ions which are innocuous in aqueous acidic
hexavalent chromium-containing solutions, there are other
innocuous ions which may be presented in certain chro
mating solutions such as barium in the case of chromat~
ing solutions for aluminum, zinc ions in the case of chro 40
perature in the range of 20° C.-22° C. and zinc sheets
'were immersed in the solution for 90 seconds and With
drawn until a total surface area of 4 sq. meters of zinc
mating solutions containing formates for chromating zinc
sheets was processed through the bath.
tration is illustrated in the following example. An aque
ous solution was prepared to contain 2.5 g./l. CrO3, 1.3
g./l. NaCl and 2.35 g./l. N03, as HNO3, and sufficient
water to make a liter.
The bath was heated to a tem
During this
surfaces. It will be appreciated that the method steps
processing the chromate content and the sodium chlo
of this invention are broadly applicable to the modi?ca
ride content were maintained constant by replenishing
tion of chromating solutions which require the presence
and sufficient nitric acid was added to offset the zinc
of a metallic cation which is normally removed upon pas 45 ions freed during the processing. At the end of this proc
sage through a strongly acidic cation exchange resin bed,
essing the bath was analyzed and found to have the fol
such as zinc ions in the case of zinc chromating solutions
containing formates.
By employing the procedure of
lowing composition: 2.45 g./l. (1103, 0.59 g./l. trivalent
chromium ions, 1.32 g./l. NaCl, ‘9.3 g./l. N03 and 2.15
g./l. Zn. The ?rst zinc sheets processed through the
regenerating the resin bed with a regenerating acidic solu
tion contaiinng a proportion of the cation which is desir 50 solution were covered with a dense light brown adherent
able in the operating chromating solution in a sufficient
coating, but after approximately 3 square meters had
quantity to saturate the resin bed with that cation, the
been processed through the bath the coating became
passage of the operating solution through the resin bed
noticeably lighter in color and did not have the denseness
will not remove that desirable metallic ion from the oper
nor adherence of the earlier formed coatings.
ating solution passed through the resin bed and thus con 55
A comparative bath prepared with the identical initial
tinuous operation will leave the operating chromating
composition and under similar processing conditions of
solution in an effective cation-balanced condition.
The most effective acidity of the aqueous hexavalent
temperature, time and type of zinc sheets, 4 square meters
of zinc surface area was processed through the bath.
During this treatment, however, a small portion of the
bath was continuously passed through a cation resin bed
exchanger containing Dowex 5O resin and the effluent re
chromium-containing solution will vary with the particu
lar metal which is being coated and such values are now
generally well known to those skilled in the art. In
general the pH can vary between 0.5 and 4.0 and as the
acidity increases, the rate of metal attack increases. With
the metals which are relatively easily attacked such as
magnesium and zinc the higher pH values are preferred, 65
whereas as the difficulty of metal attack increases the
turned to the operating solution. Dowex 50 was loaded
with H- and Na-ions. A solution of the following com
position ‘has been found suitable for regeneration:
112.5 g./l. H2804 (100%)
37.5 g./l. NaHSO;
While regenerating, the cation exchange resin'picks up,
pH should be lower, for example in the range of 1-2 for
aluminum chromating solutions. Moreover there is some
besides H-ions, sufficient of the sodium such that when
permissible variation in the acidity increase, or hydrogen
ion concentration, which can be tolerated by the operat 70 the chromating solution is contacted with the exchange
resin thus reconditioned, the pH of the ef?uent from said
ing chromating solution as the result of returning the
regenerated resin is substantially the same as the pH of
effluent from the ion exchange resin bed thereto and the
said operating solution. During this processing the coat
degree of tolerance of the operating solution varies some
ings maintained the appearance of the original panels of
what with the metal surface which is being coated. For
example, the degree of acidity variation which can occur 75 light brown colored coating which was dense and adher
3,032,447
5
out. An analysis of the solution after 4 square meters
of zinc sheets had been processed through the solution
showed that the bath contained no free trivalent chromi
um ions or zinc ions and that the nitrate ions had not
6
bed for use in conjunction with an aqueous acidic solu
tion comprising hexavalent chromium ions for forming
protective coatings on the surface of a metal selected
increased from their original concentration. During
this processing the pH value remained substantially con
stant.
The chromating of magnesium and its alloys is appro
priately done, for instance, with solutions containing
from the group consisting of Zinc, aluminum, cadmium,
magnesium, iron, copper and alloys thereof which com
prises the steps of (1) preliminarily conditioning a cation
exchange resin to contain a proportion of hydrogen ions
and metallic cations, which are innocuous to the metal
coating-forming ability of the said chromating solution
that the pH of the said chromating solution fed
about 150 g./l. Na2Cr2Oq.2H2O in water. Potassium or 10 such
through such resin is substantially ‘the same as the pH
magnesium ?uoride may still be added to that solution,
according to the saturation concentration. The degreased
of the e?luent from said resin, (2) and periodically as
said chromating solution is processed through said resin
regenerating said resin with an aqueous acidic solution
about 95° C. for about 30 minutes. Magnesium-chro
contains suf?cient of said innocuous cations such
mating baths on that base show the tendency of the pH 15 which
that
the
pH of the ef?uent from said regenerated resin is
increasing as the quantity of sheet processed therethrough
substantially the same as the pH of said chromate solu
increases, due to the magnesium ions passing into solu
tion fed thereto.
tion and the CrO3 being reduced to trivalent chromium.
2. A continuous process for forming a protective
Good dark-brown coatings are obtained if the pH is in
chromate
coating on a metallic surface by contacting the
the range of 3.5 to 5.2. When it increases above 5.2 20 metallic surface with an aqueous acidic solution compris
coatings are considerably lighter in color and of mean
ing hexavalent chromium ions, which comprises the steps
appearance.
of (1) preliminarily conditioning a cation exchange resin
Now the bath of the above composition was, in accord
to contain a proportion of hydrogen ions and metallic
ance with the invention, passed continuously over Dowex
cations other than the cation of the metal to be coated
50, loaded with H- and Na-ions by prior contact with 25 which are innocuous to the metal coating-forming ability
a solution containing 170 g./l. NaHSO4.H-2O. When
of the said chromating solution such that the pH of the
4 square meters of “Dowmetal FS 1” sheet had been proc
said chromating solution fed through such resin is sub
essed therethrough no noticeable increase of the pH
stantially the same as the pH of the effluent from said
value to the alkaline range was observed as yet. The
regenerating
said resin with an aqueous acidic solution
exchange resin had substituted for the dissolved mag 30 through the said resin as conditioned in step (1) and
nesium and the trivalent chromium an equivalent amount
continuing said cycling of solution therethrough until the
of H-ions. Continued processing of magnesium sheet
said resin is substantially exhausted, and (3) periodically
sheets are treated by immersion at a bath temperature of
resulted in some increase of the pH of the bath. Then
the exchange resin was reconditioned with a solution con
regenerating said resin with an aqueous acidic solution
contains su?icient of said innocuous cations such
taining 170 g./l. sodium bisulfate. Thus regenerated, it 35 which
that the pH of the e?luent from said regenerated resin
was contacted with the chromating bath and the initial op
is substantially the same as the pH of said chromate solu
timum of the pH=3.6 was thereby restored. Chromic
tion fed thereto.
acid was replenished throughout processing such that the
3. A process in accordance with claim 2 wherein said
chromic acid level of the chromating bath was main
innocuous metallic cations are alkali metal ions.
40
tained constant.
4. In a process for continuously operating an aqueous
For zinc chromating, solutions containing
5 g./l. CrO3 and
5 g./l. HCl (100%)
acidic chromating solution suitable for forming protective
coatings on the surfaces of metals selected from the group
consisting of zinc, aluminum, cadmium, magnesium, iron,
copper and alloys thereof in which a strong cation ex
are suitable among others. For these baths for instance 45 change resin bed is employed to remove metallic cations
Dowex 50 in mere H-form, that means regenerated with,
therefrom to thereby maintain the operating chromating
say, sulfuric acid, may be used as ion exchange resin.
solution in a substantailly steady cation-balanced condi
In a bath of that composition zinc sheets, electrogalva
nized iron sheets and zinc die castings were chromated by
immersion for 10 to 15 seconds at 20 to 25° C.
The
resulting chromate coatings were brass-colored. The vol
umes used were 1 l. of the bath and 0.25 l. of the cation
exchange resin. When processing the zinc parts, the
solution was continuously cycled through the exchange
resin at a rate of 3 l. per hour.
When 6 square meters 55
of zinc surface had been processed through, the cation
exchange resin had loaded with zinc and trivalent
chromium such that these cations began to increase in
concentration also in the bath. At that time the ex
change resin was reconditioned to its I-I-form with 15% 60
sulfuric acid and after thorough water rinsing used again
for regeneration of the chromating bath. The chromate
coatings were perfect and uniform in color on all zinc
tion, the improvement which comprises the step of peri
odically reconditioning the said resin bed with an aqueous
acidic solution containing a controlled proportion of
hydrogen ions and alkali metal ions such that the pH of
the said chromating solution fed to said bed is substan
éially the same as the pH of the e?iuent obtained there
rom.
References Cited in the ?le of this patent
UNITED STATES PATENTS
2,368,574
Gustafson ____________ __ May 29, 1945
2,733,204
Costa ________________ __ Jan. 31, 1956
2,777,785
2,864,730
Schuster et al. ________ .._ Jan. 15, 1957
Kinder et al ___________ __ Dec. 16, 1958
730,897
Great Britain _________ __ June 1, 1955
parts processed through the bath.
65
What is claimed is:
1. A process for conditioning an ion exchange resin
Shoemaker __________ .._ Jan. 30, 1945
2,376,914
FOREIGN PATENTS
UNITED STATES PATENT OFFICE
CERTIFICATE OF CORRECTION
Patent No. 3,032,447
May 1, 1962
Werner Rausch
It is hereby certified that error appears in the above numbered pat
ent requiring correction and that the said Letters Patent should read as
corrected below.
Column 1,
line 55, for "ion" read -— irorf -
line 62, before "batch" insert —— or ——; column 3,; column
line 38,2r
for "presented" read —— present ——; column 6, line 30, for
"regenerating said resin with an aqueous acidic"
—— resin,
read
(2) passing a portion of said chromating ——
Signed and sealed this 28th day of May 1963‘,
(SEAL)
Attest:
ERNEST w. SWIDER
DAVID L- LADD
Attesting Officer
Commissioner of Patents